In cellular base stations, satellite communication systems as well as other communication and broadcasting systems of today, it is often desirable to amplify multiple radio frequency (RF) channels simultaneously in the same power amplifier instead of using a dedicated power amplifier for each channel. However, when using one and the same power amplifier for the simultaneous amplification of multiple FDMA/TDMA (Frequency Division Multiple Access/Time Division Multiple Access) or CDMA (Code Division Multiple Access) channels or when using multi-level linear modulation formats, a high degree of linearity is required so that the phases and amplitudes of all the signal components are preserved in the amplification process.
If the linearity is inadequate, the simultaneously amplified channels cross-modulate, causing interference in these and other channels. The non-linearities manifest themselves as cross-modulation of different components of the signal, leading to leakage of signal energy to undesired channels. In addition, the spectra of the signal components are normally broadened, causing additional interference within the channels or in other channels. In the case of multi-level linear modulation such as QAM (Quadrature Amplitude Modulation), the non-linearities will normally lead to distortion of the constellation diagram.
The two most important properties of a power amplifier are efficiency and linearity. Consequently, the problem of enhancing the linearity must be solved while preserving high amplifier efficiency. Due to the amplitude statistics of complex multi-channel or multi-level signals, the efficiency must be kept high at all amplitude levels in order to provide high average efficiency. An additional complication is that the signals often have wide bandwidth, or wide combined bandwidth, which means that the fluctuations between high and low amplitude levels are fast.
A solution to the combined problem of enhancing linearity and preserving high efficiency has been disclosed by the Applicant in the International Patent Application WO 98/11683 and the corresponding U.S. Pat. No. 6,094,458. The proposed power amplifier uses switching techniques for the power amplification, and is based on a sigma-delta modulator in combination with a mixing and amplifying stage. The sigma-delta modulator includes a quantizer for generating an output signal with a finite number of discrete levels. The fixing and amplifying stage includes a plurality of power switches, the voltages of which are proportional to the levels of the quantizer, and the output signal of the sigma-delta modulator more or less directly controls the power switches via a decoder so as to provide appropriate power amplification. Although the proposed switch-based power amplifier has very high efficiency and can benefit from the high linearity of the sigma-delta modulator, there is still a general demand for power amplifiers with even better linearity characteristics.
U.S. Pat. No. 5,736,906 relates to a power supply modulator circuit for a transmitter. The power modulator circuit is a baseband amplifier with apportioned input impedance, buffered switching of power modules and linearizing feedback.
The International Patent Application WO 98/44626 relates to a pulse modulation power amplifier for the audio frequency range. The power amplifier comprises a pulse modulator, a power amplifier stage for amplifying the modulated signal, the output of which is low-pass filtered to obtain an analog output. The circuit further comprises a negative feedback from the power amplifier stage output to one or several loops feeding into one or several pre-amplifier stages preceding the modulator.